Subsea oil and gas exploration becomes more challenging as the exploration depth increases. Complex devices are disposed on the ocean floor for extracting the oil and for the safety of the oil equipment and the environment. These devices have to withstand, among other things, high pressures (from 3,000 to 60,000 psi (200 to 4000 bar) or more) and highly corrosive conditions. For undersea drilling, parts are disposed on the ocean floor (sometimes more than 2000 m below sea level) as shown, for example, in FIG. 1.
FIG. 1 illustrates a lower blowout preventer stack (“lower BOP stack”) 10 that may be rigidly attached to a wellhead 12 upon the sea floor 14, while a Lower Marine Riser Package (“LMRP”) 16 is retrievably disposed upon a distal end of a marine riser 18, extending from a drill ship 20 or any other type of surface drilling platform or vessel. As such, the LMRP 16 may include a stinger 22 at its distal end configured to engage a receptacle 24 located on a proximal end of the lower BOP stack 10.
In typical configurations, the lower BOP stack 10 may be rigidly affixed atop the subsea wellhead 12 and may include (among other devices) a plurality of ram-type blowout preventers 26 useful in controlling the well as it is drilled and completed. The flexible riser provides a conduit through which drilling tools and fluids may be deployed to and retrieved from the subsea wellbore. Ordinarily, the LMRP 16 may include (among other things) one or more ram-type blowout preventers 28 at its distal end, an annular blowout preventer 30 at its upper end, and multiplexer (MUX) pod (in reality two, which are referred to in the industry as blue and yellow pods) 32. Additionally, accumulator tanks 31 are provided to provide pressure to move ram blocks of the associated BOPs 26 and while shown as a separate unit, the accumulator tanks 31 can be a part of the LMRP 16 as desired.
A conventional MUX pod system 40, is shown in FIG. 2 and may provide between 50 and 100 different functions to the lower BOP stack 10 and/or the LMRP 16 and these functions may be initiated and/or controlled from or via the LMRP 16. The MUX pod 40 is fixedly attached to a frame (not shown) of the LMRP 16 and may include hydraulically activated valves 50 (called in the art sub plate mounted (SPM) valves) and solenoid valves 52 that are fluidly connected to the hydraulically activated valves 50. The solenoid valves 52 are provided in an electronic section 54 and are designed to be actuated by sending an electrical signal from an electronic control board (not shown). Each solenoid valve 52 is configured to activate a corresponding hydraulically activated valve 50. The MUX pod 40 may include pressure sensors 56 also mounted in the electronic section 54. The hydraulically activated valves 50 are provided in a hydraulic section 58 and are fixedly attached to the MUX pod 40 (i.e., a remotely operated vehicle (ROV) cannot remove them when the same is disposed on the seafloor).
In typical subsea blowout preventer installations, multiplex cables (electrical) and/or lines (hydraulic) transport control signals via the MUX pod 40 and the pod wedge) to the LMRP 16 and lower BOP stack 10 devices so specified tasks may be controlled from the surface. Once the control signals are received, subsea control valves are activated and (in most cases) high-pressure hydraulic lines are directed to perform the specified tasks. Thus, a multiplexed electrical or hydraulic signal may operate a plurality of “low-pressure” valves to actuate larger valves to communicate the high-pressure hydraulic lines with the various operating devices of the wellhead stack.
A bridge between the LMRP 16 and the lower BOP stack 10 is formed that matches the multiple functions from the LMRP 16 to the lower BOP stack 10, e.g., fluidly connects the SPM valves 50 from the MUX pod 40 provided on the LMRP 16 to dedicated components on the BOP stack or the LMRP 16. The MUX pod 40 system is used in addition to choke and kill line connections (not shown) or lines that ensure pressure supply to, for example, the shearing functions of the BOPs.
Examples of communication lines bridged between LMRPs 16 and lower BOP stacks 10 through feed-thru components include, but are not limited to, hydraulic choke lines, hydraulic kill lines, hydraulic multiplex control lines, electrical multiplex control lines, electrical power lines, hydraulic power lines, mechanical power lines, mechanical control lines, electrical control lines, and sensor lines. In certain embodiments, subsea wellhead stack feed-thru components include at least one MUX pod 40 connection whereby a plurality of hydraulic control signals are grouped together and transmitted between the LMRP 16 and the lower BOP stack 10 in a single mono-block feed-thru component.
One apparatus for sealing a well is the BOP. The BOP is a safety mechanism that is used at a wellhead of an oil or gas well. The BOP is configured to shut the flow from the well when certain well events occur. One such well event may be the uncontrolled flow of gas, oil or other well fluids from an underground formation into the well. Such well event is sometimes referred to as a “kick” or a “blowout” and may occur when formation pressure exceeds the pressure generated by the column of drilling fluid. This well event is unforeseeable and if no measures are taken to prevent and/or control it, the well and/or the associated equipment may be damaged.
The BOP may be installed on top of the well to seal the well in case that one of the above events is threatening the integrity of the well. One type of BOP, an annular BOP, is conventionally implemented as a valve to release the pressure either in the annular space between a casing and a drill pipe or in the open hole (i.e., hole with no casing) during drilling or completion operations. Another type of BOP, a ram BOP, can be located below the annular BOP and above the wellhead. The different types of rams can generally be classified as, (1) casing shear rams for cutting drill pipe, casing, etc., (2) blind shear rams capable of both sealing on open hole and cutting drill pipe, casing, etc., and (3) pipe rams capable of sealing on pipe and hanging the drill still at a tool joint.
FIG. 3 shows a ram BOP 306 located in undersea environment in more detail. A wellhead 302 may be fixed to the seabed 304, and the ram BOP 306 is secured to the wellhead 302. FIG. 3 shows, for clarity, the ram BOP 306 detached from the wellhead 302. However, the BOP 306 is typically attached to the wellhead 302. A drill pipe 308 is shown traversing the ram BOP 306 and entering the well 310. The ram BOP 306 may have two ram blocks 312 attached to corresponding pistons 314. The pistons 314 move integrally with the ram blocks 312 along directions A and B to close the well.
In situations when the ram BOP 306 is used for shearing the drill pipe or other tools in the hole, having the desired shear strength and shared load through the desired load bearing surfaces is desired. This can be complicated by variable forces acting upon the system, such as, the reaction force produced by the drill line when asymmetrically disposed relative to the shear surface of the ram block 312, and a force produced by variable upward pressure from the kick or additional items inside of the drill pipe that also need to be sheared off to seal the well, e.g., a cable attached to a down hole piece of equipment, to name just a few examples.
In order to seal the well as desired, the MUX pod 40 includes a controller which controls a system of valves for opening and closing the BOPs. Hydraulic fluid, which is used to open and close the valves, is commonly pressurized by equipment on the surface. The pressurized fluid is stored in accumulators 31 to operate the BOPs. The fluid stored subsea in accumulators may also be used to auto shear and/or perform deadman functions when control of the well is lost. The accumulator 31 may include containers (canisters) that store the hydraulic fluid under pressure and provide the necessary pressure to open and close the BOPs.
As understood by those of ordinary skill, in deep-sea drilling, in order to overcome the high hydrostatic pressures generated by the seawater at the depth of operation of the BOPs, the accumulator 31 have to be initially charged to a pressure above the ambient subsea pressure. Typical accumulators are charged with nitrogen but as pre-charge pressures increase, the efficiency of nitrogen decreases which adds additional cost and weight because more accumulators are required subsea to perform the same operation on the surface. For example, a 60-liter (L) accumulator on the surface may have a usable volume of 24 L on the surface, but at 3000 m of water depth the usable volume is less than 4 L. An additional issue with accumulators 31 is that as the charge in the accumulator 31 is expended, the resulting pressure from the accumulator 31 is reduced as shown in FIG. 4. In FIG. 4, when a valve is opened to first use the accumulator 31, the pressure generated is PI at a volume VI. As the volume of the charge is expanded, the pressure versus volume curve 402 shows that the available pressure decreases such that at a later usage point, an available pressure P2 at a volume V2 is lower than PI. This could be a problem, if the available pressure from the accumulators 31 is lower than desired.
Accordingly, it would be desirable to provide systems and methods to have a desired pressure available for use whenever desired.